The present invention relates to Pb-free soldering alloy that does not include any amount of Pb.
In the past, some Japanese patent publication laid-open such as Japanese patent publication laid-open 10-13015 and Japanese patent publication laid-open 11-277290 disclosed Pb-free soldering alloy comprising Ag, Ni and Sn and not including any amount of Pb.
The above-mentioned Japanese patent publication laid-open 10-13015 disclosed Pb-free soldering alloy comprising Ag, Ni and Sn and not including any amount of Pb but did not disclose their concrete contents.
On the other hand, the above-mentioned Japanese patent publication laid-open 11-277290 disclosed Pb-free soldering alloy comprising Ni of 0.01 to 0.5 wt. %, Ag of 0.5 to 3.39 wt. % and Sn of the balance. However, Ag of 0.5 to 3.39 wt. % was comparatively low amount and Sn was comparatively high amount to the contrary. Thus, there was a problem that much amount of Cu comprised in a printed-circuit board to be jointed by soldering was reduced to be diffused in the soldering alloy whereby resulting in lowering the mechanical strength of the jointed part of the printed-circuit board.
Furthermore, in case the amount of Ag to be added was less than 3.5% relative to the amount of Sn, the excessive amount of Sn precipitated as xcex2-Sn at the soldered part and fine cracks by the shrinkage cavity occurred on the upper layer of the soldered part during solidification.
Thus, the purpose of the present invention is to propose Pb-free soldering alloy which does not include any amount of Pb, which prevents much of the Cu contained in the printed-circuit board from being reduced to be diffused in the soldering alloy and fine cracks from occurring at the soldered part by combining Cu contained in the printed-circuit board with Ni contained in the soldering alloy and which in addition provides high mechanical strength at the soldered part.
Pb-free soldering alloy according to one aspect of the invention is characterized in that it comprises Ag of 3.5 to 6.0 wt. %, Ni of 0.001 to 1.0 wt. % and Sn of the balance.
Pb-free soldering alloy according to a second aspect of the invention includes further one or more of P, Ga and Ge of 0.001 to 1.0 wt. %.
According to the first aspect of the invention, Cu comprised in the printed-circuit board is prevented from being diffused in the soldering alloy by adding Ni to Snxe2x80x94Ag soldering alloy. Thus, in case of soldering a Cu fine line such as a lead, less amount of Cu is precipitated from the Cu fine line to the soldering alloy. As a result, the prevention of the reduction of Cu from the Cu fine line and the suppression of the reduction of its diameter can maintain mechanical strength of the lead.
In addition, as Ni is diffused in the soldering alloy, the mechanical strength of the soldering alloy itself can be kept advantageously.
Even when an electronic part and a printed-circuit board are soldered by conventional Snxe2x80x94Pb soldering alloy, Cu is precipitated from a Cu foil of the printed-circuit board and is diffused in the soldering alloy.
However, in case soldering alloy including Pb can not be used by reason that Pb causes an environmental problem and Pb-free soldering alloy including Sn principally is used instead, more amount of Cu is precipitated than in Snxe2x80x94Pb soldering alloy and diffused in the soldering alloy because the more amount of Sn is included in the soldering alloy, the more easily Cu is precipitated and diffused in the soldering alloy. Thus, the Pb-rich soldering alloy has been used inevitably to prevent much amount of Cu from being precipitated and diffused in the soldering alloy.
From the above-mentioned reason, Snxe2x80x94Agxe2x80x94Cu soldering alloy and Snxe2x80x94Agxe2x80x94Cuxe2x80x94Bi soldering alloy reduce the mechanical strength of a lead to be soldered because the principal constituent of them is Sn and they cause the precipitation and the diffusion of more amount of Cu than the conventional type soldering alloy.
According to the present invention, as the amount of Ag included in the soldering is 3.0 to 6.0 wt. % which is more than in the conventional type, the amount of xcex2-Sn in the soldering alloy is less. Owing to the multiplicative effects of the presence of fine Ag3Sn and the diffusion of Ni, the mechanical strength of the soldered joint is raised.
In addition, We found that the addition of Ni prevented Cu of the Cu foil on the printed-circuit board from being precipitated and diffused in the soldering alloy.
According to the second aspect of the invention, as any one or more of P, Ga and Ge is or are added to the soldering alloy by 0.001 to 1 wt. %, the oxidation of the soldering alloy during soldering is reduced, the generation of dross is suppressed and good soldering capability is given.